Atmospheric noise at VLF/LF is highly impulsive and non-stationary over any time interval greater than 10 to 15 minutes. Because of the excellent propagation of energy in this frequency range the noise, which is produced by lightning discharges, is characterized by high dynamic range impulses on the order of 60 dB or more over that of receiver internally generated thermal noise. The noise in general, can be described by a low amplitude Gaussian-type background noise in combination with that of the impulsive noise. Because of the non-stationary characteristics of VLF/LF atmospheric noise, attempts have been made in the past to simulate its characteristics in such a manner that communication systems could be evaluated with repeatability. Atmospheric noise generators to date have not been designed to provide the noise with the temporal characteristics of "real world" atmospheric noise.
The design concept for a forerunner of the present inventive concept is depicted in FIGS. 1 through 3 of the drawings and is fully disclosed in U.S. Pat. No. 4,173,000 issued Oct. 30, 1979. This design has a continuous impulse noise amplitude probability distribution digitally estimated in a piece-wise fashion in 6 dB amplitude increments. These increments are linearly combined with a digitally generated Gaussian noise component for simulation of the VLF/LF atmospheric noise.
Description and operation of the prior art noise generator calls for its being set for a 72 dB dynamic range using 12 lines of input to a digital-to-analog converter (DAC). A clock is used to drive seven pseudo random-bit generators (PRBGs) of linear feedback type. One of the PRBGs is driven directly by the clock while the other six are driven by the clock signal after being divided down in the divide-by-N circuit. From the slower PRBGs various lines from each stage are wired to the 12 programmable probability gates. Only one line from each of the slower PRBG stages is used for input to each of the programmable probability gates. No more than six lines from any PRBG goes to any programmable probability gate. The progrommable probability gate is a hard wired logical device used to convert the outputs of the PRBGs into pulses which will occur at a specified probability. For example, the output of the PRBGs is in the form of binary logic level voltages which are denoted as "1" or "0". The "1" or " 0"s from PRBG lines to that particular programmable probability gate shown in FIG. 3 are used in two ways. First, the lines are input to a "nand" gate. These inputs are nanded together having the resultant output enabling the MUX "M". All inputs have to be at a logic "1" for the output of the nand gate to be a logic "0"; else the output is a "1". If the enable input to the MUX "M" is a "1", the output is a zero. If the input is a "0", the output of MUX "M" is determined by the data inputs and the data selected input. The data select input comes from individual taps from the stages of the slower PRBGs. The output when the enable is zero is determined by the table below.
______________________________________ A B C Data Selected ______________________________________ 0 0 0 0 0 0 1 1 0 1 0 2 0 1 1 3 1 0 0 4 1 0 1 5 1 1 0 6 1 1 1 7 ______________________________________
One of the data input lines is the output of MUX "N" while the remainder come from the header socket.
MUX "N" is similar to MUX "M" but with all data inputs coming from the header socket, the enable set to a fixed logic "0", the data select lines come from the slower pseudo random sequence generator (PRSGs), and the output going to one of the "data input lines of MUX "M". Note the programmable nature of the gate comes from the fact that the number of logic "1"s or logic "0"s at the input to either MUX "M" or MUX "N" can be altered by wiring a header which will fit into the header socket. Thus, if the number of inputs to the nand gate is denoted by "K", the number of logic "1" inputs to MUX "M" is denoted "M", and the number of logic "1" inputs to MUX "N" is denoted "n", the probability of a logic "1" output is given approximately by ##EQU1## where EQU 0.ltoreq.K EQU 0.ltoreq.m.ltoreq.7 EQU 1.ltoreq.n.ltoreq.8
In FIG. 2, the output of each of the 12 programmable probability gates goes to a corresponding one-shot multivibrator and then to the digital-to-analog converter or DAC. Assume the least significant input of the DAC when pulsed by the one-shot produces an output level of A, then the output is a random series of pulses ranging in amplitude from A to A X (2.sup.n -1) where n equals the number of address lines used in the DAC.
The output of the DAC and the output of the fast PRSG, which is Gaussian in nature, are inputs to the summing bus where Gaussian noise is linearly summed with the impulsive noise. The ratio of the Gaussian noise relative to impulsive noise is a function of two fixed resistors in the summing network.
The output of the summing network is then passed through a low pass filter to limit the bandwidth of the output. Now that the energy has been confined to a limited bandwidth it can be amplified by a wide dynamic range amplifier to produce the desired output at a convenient level.
The prior art system and method of pseudo-atmospheric noise generation though a noteworthy advance in the state-of-the-art did not fully realize several features. First, there was no control of the temporal characteristics of the impulse noises. Secondly, although not readily apparent the prior art unit required a sophisticated noise analyzer for calibration, self-testing, or setting up of signal-to-noise ratio for modem testing. And, thirdly, the old generator could produce only one amplitude probability distribution at a time from the programmable probability gate settings.
Thus, there is a continuing need in the state-of-the-art for an improved pseudo-atmospheric noise generator that has the flexibility and capability for changing its temporal characteristic and amplitude probability distributions to reflect disturbances under a variety of changed circumstances.